The biomechanical requirements for electronic mouse operation may be risk factors for cumulative trauma disorder (CTD) of the forearm and wrist and may limit skill acquisition and ultimate skill proficiency among both highly skilled and novice mouse users. No commercially available mouse eliminates all of these constraints, and studies of mouse user proficiency have not adequately assessed skill acquisition, especially with respect to the musculoskeletal system. The objectives of this study are; 1) to evaluate the biomechanical effects of mouse use on the forearm and wrist, 2) to design an alternative mouse that reduces the risk of forearm and wrist (CTD and to determine the biomechanical effects of mouse design, and 3) to determine the effect of mouse design on skill acquisition and proficiency. The hypotheses to be tested are as follows: 1) use of a standard mouse will be associated with fixed wrist extension, fixed wrist ulnar deviation, use of wrist radial- ulnar deviation for mouse operation, and continuous, submaximal electromyographic (EMG) activity by the forearm pronators and wrist extensors and ulnar deviators; 2) the new mouse design will reduce these biomechanical constraints; 3)novice mouse users will demonstrate an increased rate of skill acquisition with the new mouse design as indicated by improvement over time in the skill level, speed, accuracy, and motor coordination pattern with which a mouse input task is performed; and 4) both highly skilled and novice mouse users will demonstrate better ultimate skill with the new mouse design. One-hundred twenty-five skilled and novice occupational mouse users will be tested five times over seven weeks on a graphical mouse targeting task using a standard mouse and a new mouse designed for this project. 3D kinematics and EMG of the forearm and wrist will be measured on three of the visits. A two factor hierarchical ANOVA will be used to compare ultimate mouse skill across user skill category and mouse design. ANCOVA will be used to compare the rates of mouse skill improvement across user skill category and mouse design. Student's t-tests will be used to compare the fixed postures and ranges of motion required for mouse use across mouse design. Kinematic and EMG patterns will be used to compare changes in motor coordination patterns across skill category and mouse design. The study will document the extent of CTD risk factors among occupational mouse users. It will result in a new mouse designed to reduce these risks. It will explore the impact of the constraints of mouse use on the acquisition of a complex functional task. it will investigate the effectiveness of an ergonomic solution intended to balance the problems of worker risk and the upper limits to skilled worker performance. Such knowledge will foster the prevention of CTD in computerized offices and enhance understanding of the limits to skill imposed by constraints in the human-machine interface.